Professional Monitor MkIII

IMF Professional Monitor Mk. III


The Professional Monitor Loudspeakers were the first to be designed and produced by IMF Products in England. All other speakers in the IMF range are modelled from knowledge gained in the furtherance of research for the professional monitor series. The IMF Professional Monitors epitomize the ultimate state-of-art approach to loudspeaker design. Individually hand-crafted, irrespective of the costs involved, they are produced at a price rather than to a price.

The MONITOR MK, III is the culmination of a generation of involvement in "state of the art" transducers. Individually hand-crafted in I M F's English laboratories, it is a no compromise design for use wherever the requirement is reference standard reproduction.

A "Monitor" loudspeaker is a laboratory tool. It must be consistent in performance, designed to be as independent as possible from interaction with the listening environment; and preferably should have the absolute minimum of "coloration", or "loudspeaker sound". For these reasons, such popular designs as corner horns, reflector speakers, or omni-directional radiators are never used in sound studios or for analysis of program inputs.

Since the emergence of plastic cone technology in monitoring loudspeakers, owing to the long term researches of the BBC, monitoring loudspeakers have been designed as multiple drive, plastic diaphragm dynamic loudspeakers.

The term "Monitor" then refers to a degree of performance, rather than to "listening pleasure", which is generally the goal of the home loudspeaker. Indeed, a "Monitor" may often reveal all too clearly a poor program, or an inferior cartridge. However, over a range of program material, it has been shown by countless hours of psychoacoustic research, a true "Monitor" will be more aesthetically satisfying than a home loudspeaker; and on the best program material, a "Monitor" will offer breathtaking realism. This is why IMF MONITOR loudspeakers are in use in homes throughout the world; music lovers, once having experienced true Monitor performance, cannot settle for less!

The MONITOR III, latest in the MONITOR series, is a large loudspeaker - because a loudspeaker intended to reproduce a wide band, uncolored sound image from a range of moderately powered amplifiers must be large. The size is purely a matter of physics, relating to band pass, efficiency, coloration, and the acoustical laws of room propagation. The MONITOR III is a "fourth generation" refinement of the now famous transmission line principle (first generation, the stuffed line of 1965; second generation, the tapered line of 1968; third generation, the "flow through" damping of 1969). The "fourth generation" is the development and use of "honeycomb mesh damping" (see illustration), which effectively doubles the absorption area of the filters and increases the acoustical path length. Each "honeycomb mesh" filter must be handwoven into the transmission line.

The effect is to increase the performance gap between the modern transmission line and all other room sized enclosures. The MONITOR III is the first and only room sized loudspeaker which can reproduce the wave envelope of any bass instrument in its original shape and time sequence! (Heretofore, 25 foot concrete horns were used by those who wanted such accuracy). For those familiar with the live sound of the lowest pedal notes of the organ, double basses of the orchestra, bass drum, tympani, cello, bass tuba, one audition of the MONITOR III ends all discussion of the merits of other loudspeakers!

Carried on in the MONITOR III are the now accepted virtues of "plane source propagation", separate line for the vital mid band, and "inward dispersion"; Because of the addition of the "honeycomb mesh" system, it has been possible to improve the mid band radiation patterns over the lofty standard of the MONITOR II, by increasing the size and critical damping of the mid line. Since a phase perfect mid band reproduction is the sine qua non for proper perception by the critical human ear, the redevelopment of the mid line has resulted in an even clearer view of the original program.

The treble bank is two units, extending the range well into the supersonic region, with unparalleled low distortion, smoothness, and power handling. The treble bank and mid bank are critically phase spaced at the inner boundaries of the enclosures (the speakers are built in "mirror image" pairs). A word of explanation: For proper location of the stereo image, phase and intensity relationships must be accurately maintained. It is impossible to maintain proper phasing between any multiple drivers over more than a specific and limited angle of rotation. The usual loudspeaker design ignores the problem the IMF design does not! The drivers in each enclosure are so arranged that optimum dispersion and phasing occur inward, toward the listener, forming a stereo image of vast depth and precision (the "outer" dispersion, out of phase, is dispersed from the room boundaries).

IMF MONITOR III - individually Crafted from high density building board and fully laminated in formica overlay. Fitted castors.

Dimensions approx. 42" x 17 1/2" x 19 3/4" wide.

Bass unit 13" x 9 1/2" flat polystyrene diaphragm, mid-range 6" plastic cone, tweeter 1 3/4" chemical diaphragm and 3/4" chemical dome super-tweeter.

Crossover electrical 4 way at approx. 375 Hz, 3.5 kHz and 13 kHz. Level controls provide subtle adjustment of approx. ±2dB over mid-range and tweeter to accommodate variations in room reflection coefficients.

Frequency range from 10 Hz to beyond audibility.

Driving power requirement 25 to 100 watts under speech and music conditions.

Efficiency measured via pink noise 1 metre on axis: 25 watts produces 100 dB.

Nominal matching impedance 4-8 ohms.

Supplied to order in matched 'mirror image' pairs with solid aluminium facia trim and finished in overlay specified which can include wood facsimile laminates and standard plain colours.

Weight approx. 140 lbs.

Subject to alteration without notice.



Each MONITOR III is enclosed in a stressed formica skin, laminated over wood with a viscous glue dampener. Ordinary wood is highly translucent to sound waves, passing them freely. The laminating technique in the MONITOR III reduces cabinet feedthrough to negligible proportions. The design techniques employed in the MONITOR III result in the specifications shown, far surpassing any previous monitoring standards. How does such a meticulously conceived loudspeaker sound? Truthfully, the MONITOR III does not "sound" like any other loudspeaker available (we exclude the custom built ribbon and very long concrete horns with which it has been compared by the English critics). One critic describes a "breathy quality down to the subsonic region (below 20HZ), and an even better analysis giving quite remarkably detailed results on the best stereo programmes. . .


Measurements taken of samples under anechoic conditions with reflection coefficient better than 0.1 show the normal characteristics.

Equipment employed B & K pen recorder, noise and signal generator, third octave filters, polar turntable and microphones.

Checking recent good discs I have noted particularly the feeling of 'listening through' the audio chain, such is the open quality and revelation of detail" (complete review on request). Another critic talks of “. . . the transient performance, which was clear and capable of jolting the listener upright at the appropriate moments ... a balance less bright than some modern loudspeakers . . . a very 'full' sound due to some extent to the undoubtedly extended bass response . . . uncanny realism on first-class signals" (complete review available).

Little can be added to the above comments. The quality of reproduction is so different from conventional loudspeakers that it cannot be compared. With the proper program, the proper disc and cartridge, the reality is indeed awesome. We suggest you listen to a modern recording of the opening, Mahler

Second Symphony; or the introduction to the Fourth Movement of the Beethoven Ninth; or any disc with bass drum, organ, celli, violin, piano; and especially to the human voice, either classical or modern. The voice, most difficult of all "instruments" to reproduce naturally and in its full force, comes effortlessly through the MONITOR III to your ears.

What is lacking, in the MONITOR III, is any suggestion of "hi fi", of bloated strings or piano, of seven foot high voices - the unfortunate effects of some of today's misguided loudspeakers.

The MONITOR III, now firmly established as the "reference standard" of the world, is sold in the U.S. only through IMF dealers specially qualified and franchised for the purpose. Standard finish is walnut formica; other finishes are available on special order. The MONITOR III is unconditionally guaranteed for five years, under normal use conditions. Full technical data on normal use conditions, including recommended auxiliary equipment, are available from the supplying dealer.

Price: $850 each, $1750 the Pair, F.O.B. Phila.

warehouse $1700


7616 City Line Ave.

Phila Pa 19151 USA

(tele. 215-473-7474)

Printed in U.S.A.